Tools

Tag: Scanner

This example show you how to validate input when using java.util.Scanner. To validate input the Scanner class provides some hasNextXXX() method that can be use to validate input. For example if we want to check whether the input is a valid integer we can use the hasNextInt() method.

In the code snippet below will demonstrate how to validate whether the user provide a positive integer number. The program will repeat until the correct input is supplied.

Please enter a positive number: qwerty
"qwerty" is not a valid number.
@@@
"@@@" is not a valid number.
-100
Please enter a positive number: 99
You have entered a positive number 99.

Another example is to validate if user correctly input letters to guest a secret word. In the code snippet below if the user does not enter a letter the code will keep asking for a valid letter. It loops until the length of the inputted letters equals to the length of secret word.

Enter a letter to guess: 1
Enter a letter to guess: 2
Enter a letter to guess: H
Enter a letter to guess: e
Enter a letter to guess: l
Enter a letter to guess: l
Enter a letter to guess: o
You are correct!

Here is a compact way to read file line by line using the java.util.Scanner class.

package org.kodejava.example.util;
import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
public class ScannerReadFile {
public static void main(String[] args) {
// Create an instance of File for data.txt file.
File file = new File("data.txt");
try {
// Create a new Scanner object which will read the data
// from the file passed in. To check if there are more
// line to read from it we check by calling the
// scanner.hasNextLine() method. We then read line one
// by one till all lines is read.
Scanner scanner = new Scanner(file);
while (scanner.hasNextLine()) {
String line = scanner.nextLine();
System.out.println(line);
}
} catch (FileNotFoundException e) {
e.printStackTrace();
}
}
}